What is the Heat of Hydrogenation? |Examples

Hydrogenation is the reaction of an unsaturated compound with hydrogen in the presence of a catalyst. This reaction can saturate a compound. The amount of heat that evolved in this reaction is called Heat of Hydrogenation.

Definition:

Reaction involves to reduce an unsaturated compound to a saturated compound by adding hydrogen..

or,

The Reaction of an unsaturated compound with hydrogen in the presence of metal catalysts to form a saturated compound is called catalytic hydrogenation.

Catalysts Used:

The most commonly used catalysts are platinum(Pt), palladium (Pd) and Nickel (Ni).

  • Platinum is used as PtO.
  • Platinum is employed as fine powder supported on charcoal
  • Nickel is used as Raney nickel. (Raney nickel is obtained by dissolving Ni-Al alloy in NaOH which dissolves Al leaving Ni as a fine suspension, called Raney nickel ).
Hydrogenation reaction
H

Example of a Hydrogenation reaction:

Alkenes react with hydrogen under pressure and in the presence of Ni ,Pt or Pd catalyst to form alkanes.

Catalytic hydrogenation of alkenes:

The addition of hydrogen to an alkene is cis. the hydrogenation of alkenes occurs at the surface of a metal catalyst. Metals absorb hydrogen by providing electrons to form metal hydrogen bonds. at the same time, an alkene is also adsorbed on the surface of the metal breaking the “pi” bond of alkene and helping in a step-wise transfer of hydrogen atoms to the alkene to produce the corresponding alkane which then leaves the surface of the metal.

 view of laboratory hydrogenation reactions. The heat of Hydrogenation.

Applications of Hydrogenation:

Applications of hydrogenation reactions are:

  • Hydrogenation is basically used to saturate a compound.
  • Hydrogenation is quantitative so we can use this to find out the number of double bonds in Polyalkene by measuring the volume of hydrogen used.
  • Catalytic hydrogenation is used on an industrial level to produce vegetable ghee from vegetable oil.
  • This process is used for the treatment of various products such as ammonia alcohols pharmaceuticals margarine polyols polymers and some chemicals also.
  • Heat of Hydrogenation helps us to determine or find out the stability of compounds.

Heat of Hydrogenation:

It is defined as;

Amount of heat evolved when one mole of alkene is hydrogenated.

  • Heat of Hydrogenation of most alkenes is 126 kj for each double bond in a molecule

Example:

Hydrogenation of isomeric alkene,1-butene,cis-2-butene, and trans-2butene all these three add one molecule of hydrogen to produce the same product n-butane.

1-butene has largest amount of heat of Hydrogenation trans -2-butene has least amount of heat so this one is more stable because of low heat of hydrogenation.

Another example of ismomeric methyl butenes is as follows;

  • 2-methyl -2-butene consisting 3 alkyl groups on double bond is more stable.
  • 3-methyl-1 butene contains 1 alkyl group on double bond is least stable.

It will show that stability of order of reaction of these alkenes depends on the number of alkyl groups attached to carbon atom of double bond.

Stability order of alkenes is as follow:

Tetra-substituted> Tri-substituted > Di-substituted> Mono-substituted> Un-substituted.

People ask:

Which has more heat of hydrogenation?

Stability and heat of hydrogenation have an inverse relation. Higher the value of heat of hydrogenation more energy is released and that makes the breaking of the double bond easy and the double bond is less stable.

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